Objective: The objective of this feasibility study was to evaluate the acceptability and potential effectiveness of a web-based MMR decision aid in supporting informed decision-making for the MMR vaccine. Design: This was a prospective before-and-after evaluation. Setting: Thirty parents of children eligible for MMR vaccination were recruited from two childcare organizations in a moderately deprived community in a city in the north of England. Method: Parents completed postal questionnaires at recruitment, one week and three months after using the decision aid. Data on the acceptability of the decision aid, informed decision-making outcome measures and vaccine uptake were collected. Telephone interviews were conducted with five parents. Acceptability data were analysed using simple descriptive statistics. Repeated measures ANOVAs were undertaken for analysing the informed decision-making data. Content analysis of the qualitative data was undertaken. Results: Twenty six parents (87 per cent) completed all three study questionnaires. The decision aid was found to be acceptable to parents and was considered useful in supporting their informed decision-making. There was a statistically significant increase in parents' knowledge over time and statistically significant decrease in decisional conflict for the MMR decision. Most parents (88 per cent) reported vaccinating their child. Conclusions: We cannot extrapolate from our data to infer that the decision aid has changed decision-making outcomes or influenced vaccination uptake. However, findings suggest that it may increase knowledge about MMR and reduce decisional conflict to a level where parents can make an informed decision. A randomized controlled trial is now underway to expand on these findings. (Contains 2 tables.)